U.S. Pat. No. 3,515,798, assigned to the assignee of the present invention, describes an elastomeric cover and removable core assembly which is particularly useful in the electrical distribution industry. However, the application of rubbery insulating sleeves to electric wire or cable splice areas is illustrative, and although the invention will be described primarily in terms of devices and procedures adapted particularly therefor, it is not to be construed as limited thereto, being equally applicable, for example, in the application of corrosion-preventing protective sleeves to welded pipe joints. The assembly is typically referred to as cold shrink tubing to differentiate it from polymeric tubing which may be shrunk by the application of heat.
FIG. 1 illustrates a typical use for and construction of a cold-shrink tube assembly and shows two cable-ends 11 comprising a stranded conductor 12 and an insulating covering 13. The covering is cut away at 14 and the conductors 12 joined together in end-to-end configuration by suitable means which may typically consist of a compressed or indented metal sleeve or a close-fitting metal tube with set-screw insulating mastic or tape, here omitted for clarity of illustration.
The cold-shrink tube assembly is slipped over one of the wire-ends prior to joining the two ends. After the splice is completed, the assembly is slid into position over the splice area and the support is removed to permit the elastic cover to contract and form a tight fit. The process will be apparent from the illustration. The support comprises a unitary tubular core 15 helically grooved along its entire length, the continuous groove 16 permitting the core 15 to be pulled out into a continuous strip 17 which is removed through the bore, i.e., from between the core and the cable 11. An elastic tube 18 in radially extended or stretched condition is supported on the core 15. As the strip 17 is progressively withdrawn, the tube 18 contracts about the cable as at 19 to form a closely conforming and tightly retained protective covering. Contraction of the tube results in the application of a resultant force against the end of the core 15 and assists in the removal of the strip 17 as the core 15 is unwound.
Although the construction described above has been used effectively for many years, considerable effort has been invested to reduce the amount of material used for the core 15 without compromising the strength of the core 15, i.e., its ability to withstand the compressive forces imposed upon it by the elasticity of the tube 18.
One method of reducing the amount of material used in the core 15 has been to produce the core 15 from a continuous ribbon 20 such as that shown in FIGS. 2 and 3. The ribbon 20 includes edges 22 and 24 which interlock, as shown in FIG. 3, when the ribbon 20 is helically wound to form a tubular core. The interlocked edges 22 and 24 may be joined by such means as adhesives, heat welding or solvent welding, but the preferred method is ultrasonic welding. The construction of FIGS. 2 and 3 was effective to reduce the amount of material used in the core 15 since the thickness of the core tube could be reduced as it was no longer necessary to cut a groove 16 in the material to form the helical line of weakening which allowed the core 15 to be pulled as a strip 17 from the assembly. The joint between the edges 22 and 24 of the joined ribbon 20 formed the helical line of weakness around the core 15. Unfortunately, it was found that the extensive surface area of the contact between the two edges 22 and 24 of the ribbon 20 resulted in bonds at the joint surface which were difficult to control, both in terms of location and strength. As a result, the core 15 was at times too weak to support the elastomeric sleeve 18 or too strong to allow easy stripping of the core 15 from the sleeve 18.
The present invention modifies the shape of the ribbon edges 22 and 24 in order to achieve greater uniformity of bonding at the joint.
As less material is used in the support core, the possibility of premature collapse increases, especially as the diameter of the core becomes larger. The support core must have strength sufficient to resist collapsing under the compressive force of the tube for long periods of time and at elevated temperatures. The external pressure of the tube can cause collapsing of the support core, for instance, by buckling. This effect can be enhanced by the uneven thickness of the expanded tube which results in uneven pressure on the support core. Subject to this uneven pressure, the support core takes on an oval shape which is easier to collapse than a perfect circular cylinder. Ellipicity is the most important defect which determines premature collapse of circular supporting tubular cores. As the diameter of the support cores and expanded elastic tubings increases, the defects may become more pronounced and thus make premature collapse more common. Further, for given materials, the thickness of support cores required to support large diameter expanded elastic tubings is a fast function of the diameter of the core. Hence, there has been a natural limit to the size of tubings which can be reliably supported by collapsible support cores without the wall thickness of the core becoming unacceptably large.
The invention provides a ribbon with greater hoop strength by placing within the ribbon a support member formed of a material capable of withstanding high temperatures and increased pressure associated with large diameter support cores and stretched elastic tubings.